Question

A human heart pumps blood of 70ml for each beat with a pressure of 12mm. Calculate it's power if the number of beats per minute is 72. 

Answer 1

Power = force×velocity
          =(pressure×area)×(distance travelled per unit time)
          = pressure × (area×distance travelled) per unit time
          = pressure × volume per unit time

No. of beats per minute = 72
beats per second = 72/60= 6/5 = 1.2
volume pumped per second = 70×1.2 = 84 ml = 8.4×10^(-5)m³
pressure = 12 mm Hg = 1600 Pa

Power = 1600×8.4×10^(-5) = 0.1344 J/s

Answer 2

I take pressure given as pressure in terms of mm of mercury column.

Power = energy / time 
           = Force * distance / time = Pressure * area * distance / time
          = Pressure * volume / time 
          = 12 /1000 m * 13,600 kg/m² * 10 m/sec² * 70 /(1000 *1000) m³ * (72 / 60 sec)
          = 12 * 13600 *10 *70 * 72 / (1000*1000*1000*60)  watts
          =  137.088 milli watts
          = 0.137 Watts
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Volume of blood pumped from heart = blood flow rate 
            =  70 ml * 72 / 60 sec = 84 ml / sec

Pressure = 12 /1000 m * 13,600 kg/m² * 10 m/sec² = 1632 N/m² or Pa

Power = pressure * flow rate = 1632 * 84 = 137.088 milli Watts